Semiconductor lasers of the type mentioned above have, for example, become important components in the technology of optical communication, particularly because such lasers can be used for amplifying optical signals immediately by optical means. This allows to design all-optical fiber communication systems, avoiding any complicated conversion of the signals to be transmitted which improves speed as well as reliability within such systems.
In one kind of optical fiber communication systems, the lasers are used for pumping erbium-doped fiber amplifiers, so-called EDFAs, which have been described in various patents and publications known to a person skilled in the art. Examples of technical significance are three main types, typically used for erbium amplifiers, corresponding to the absorption wavelengths of erbium: strained quantum-well InGaAs lasers are used at 980 nm; GaAlAs lasers at 820 nm, and InGaAsP and multiquantum-well InGaAs lasers at 1480 nm. The present invention is especially directed towards the latter type of lasers, i.e. lasers for wavelengths of more than 1100 nm and may for example be implemented in InGaAsP, multiquantum-well InGaAs lasers, or AlGalnAs/InP at 1480 nm.
To achieve the desired results with such laser diodes, a low vertical farfield of the laser's exit beam is considered advantageous, associated with a large vertical extension of the nearfield to obtain low optical power density in the laser as well as enhanced coupling efficiency for fiber coupling. In addition, low internal losses are essential for efficient high power operation. However, it is not an easy task to achieve this for the group of lasers here addressed.
Here, some remarks on the technical realization of such laser diodes seem appropriate. Persons skilled in the art distinguish between the optical nearfield, i.e. the optical mode in the laser cavity, and the optical farfield, i.e. the mode outside the laser. The lateral dimension of the nearfield is determined by the lateral structure of the laser diode. This is done to avoid laser operation in higher order modes. The lateral dimension is usually several micrometers, typically between 2 to 5 micrometers. Due to the difficult realization of efficient laser structures with large vertical optical nearfields, the dimension in the vertical direction typically is much smaller, usually below 1 micrometer, often in the range of tenths of micrometers.
Since the vertical dimension of the nearfield is much smaller than the lateral dimension, the corresponding farfields have large vertical farfield angles, while their lateral angles typically are much smaller. This is a consequence of the diffraction of the beam when exiting the laser cavity. As a result, in waveguide lasers of the type discussed here, the nearfield generally has the shape of a horizontally extending ellipse, whereas the farfield has the shape of a vertically extending ellipse. This situation is based on a horizontally located laser as shown in FIG. 1.
Optical fibers, the other essential component of fiber networks, generally have a circular cross section which unfortunately does not match the elliptic cross section of the farfield. Significant efforts have been made to avoid the losses occurring by this mismatch, because finally the power coupled into the fiber—and not the “raw” power of the laser diode—determines the performance of a fiber network. One of these efforts is to improve the coupling between the laser's farfield and the fiber. Shaping the form of the farfield thus appears to be a particulary promising approach.
To match the cross section of the optical fiber, a reduction in the vertical farfield is beneficial. This can be achieved by realizing a large vertical nearfield. Such a large vertical extension of the elliptic optical nearfield can be obtained by weakly guiding the optical mode in the vertical direction. To obtain a high efficiency especially for long cavities which are essential for good heat removal, it is necessary that the laser structure has low internal optical losses. For InP-based material systems it is known that the main contribution to optical losses arises from free carrier absorption predominantly in the p-type doped regions. Therefore, the mode is preferably guided in the n-doped part of the laser structure in an asymmetric waveguide arrangement.
To guide the mode in a large waveguide, a small contrast between the indices of refraction of the waveguide layer and the surrounding (cladding) layers is required. In InP based materials, it is beneficial to choose cladding layers made of InP due to the improved electrical and heat conduction of binary alloys compared to ternary or quaternary alloys. To obtain a small contrast for the index of the waveguide to the cladding layers, a relatively low index quaternary material is required. This is characterized by the photoluminescence wavelength (λPL) which needs to be below approximately 1000 nm (1.0 μm).
A drawing of such a structure is shown in FIG. 1, explained further below. It is known that the composition of thick layers of these materials is difficult to control. Lack of control easily leads to distortions of the resulting mode profile, degrading the performance of the laser diodes and significantly reducing the yield of the manufacturing process. Therefore, the low index waveguide is made of a set of materials which allow an easier control, like for example InP and a material with a λPL of 1100 nm (1.10 μm) or larger, usually referred to as Q1.10 material. The required average index of refraction for the waveguide can now easily be obtained by appropriate variation of the thickness of a pair of InP and Q1.10 material layers.
Another problem associated with weakly guided modes in waveguides having a large extension is that these waveguides are potentially multimode guides, i.e. allow for the generation and guiding of higher order modes. With the solution according to the present invention, it is possible to shape the mode profile for the zero order and higher order modes using an arbitrarily graded index profile enabling discrimination of the higher order modes in potentially multi-mode vertical waveguides.
A third problem is associated with weakly guided modes in asymmetric waveguides. Asymmetric waveguides are advantageous since most of the intensity of the mode is guided in the n-doped part of the section where free-carrier absorption is less dominant. Often asymmetric waveguides are realized taking benefit of an additional small waveguide embedded in the n-cladding of the laser structure nearby the active waveguide. While increasing the size of the mode in such an asymmetric waveguide, these waveguides tend to be unstable against changes in the index of refraction (e.g. due to heating or change in carrier density) since the additional waveguide needs to become very large and in a significant distance to the active waveguide. Using the approach of the large optical superlattice (LOSL) of the present invention, the mode is guided much more stably since the effect of the additional waveguide is distributed over a large waveguide with an averaged low index contrast to the cladding.
A number of solutions have been tried to solve the problems above. None of these solutions, however, worked satisfactorily. For a better understanding, some of the most relevant solutions tried are described below.
One solution, described by T. Namegaya, R. Katsumi, et al in IEEE Journal of Quantum Electronics, V. 29 No. 6, June 1993, pp 1924–1931 under the title “1.48 μm high-power GaInAsP-InP graded-index separate-confinement-heterostructure multiple-quantum-well laser diodes”, uses symmetric small waveguides. These are waveguides with a dimension much smaller than the material wavelength. They loosely guide the mode and can be used to realize large nearfields and hence small farfields. Unfortunately, these waveguides exhibit high optical losses associated with the high free-carrier absorption from the overlap of the mode with the p-doped material. Therefore, in InP based material systems, these lasers typically show low efficiency.
Another known solution uses symmetric large waveguides containing low-index material as described by M. Maiorov et al. in Optical Fiber Communication Conference and Exhibit, 2001. OFC 2001, V. 3, 2001, pp.WC2-1-3 vol. 3, entitled “High power InGaAsP/InP broad-waveguide single-mode ridge-waveguide lasers”. These are undoped waveguides with a dimension much larger than the material wavelength; they again loosely guide the mode and can thus be used to realize large nearfields and hence small farfields. Although optical losses in the undoped waveguides are low, these waveguides are not very suitable for the following reasons:                the control of the low index material is difficult;        due to the large undoped regions, additional unwanted voltage drops occur;        for lasers which should run in lateral single mode operation, it is required to etch into the waveguide to obtain sufficiently strong lateral guiding. This leads to both manufacturing problems (since control of etch depth for example in ridge waveguide lasers is difficult) and reliability issues (due to the higher concentration of carriers at the etched surface of the laser).        
A further solution are asymmetric waveguides using an additional waveguide as shown in U.S. patent application Ser. No. 10/141,914 by B. Reid et al, assigned to the assignee of the present invention. These devices have an additional waveguide most beneficially placed in the n-part of the laser structure enabling reduced optical losses. The additional waveguide works as a slight disturbance for the mode and is therefor limited to a reduced thickness and a distance from the active waveguide disabling to reach very low farfield angles. At extended size and distance of the additional waveguide, such structures are subject to instabilities leading to degradation of the performance, especially when the index is changed, e.g. due to heating or change in carrier density.
A still further solution tried digital alloys and small electronic superlattices. The idea was that a low average index of refraction can be obtained by alternating layers (most often binary or ternary alloys) with a thickness on the order of the de-Broglie wavelength of the carriers (electrons) in the semiconductor material, resulting in small electronic superlattices. This can also be obtained by alternating the layers on an extremely fine scale, the thickness being in the range of monolayers, i.e. by making a digital alloy as described by A. Ginty et al in “Long wavelength quantum well lasers with InGaAs/InP superlattice optical confinement and barrier layers”, Electronics Letters, V. 29, No. 8, 15 April 1993, pp 684–685. Both methods are used to produce an average electron property, i.e. an average bandgap energy. In addition, the first method is typically carried out as a resonant structure providing an additional energy gap for carriers as shown by R. V. Chelakara et al in “Enhancement of potential barrier height by superlattice barriers in the InGaAsP/InP materials system”, Electronics Letters, V. 31, No. 4, 16 February 1995, pp 321–323. However, none of the two methods is suitable for the much larger dimensions required for optical guiding. This is due to the large number of interfaces associated with this technique, interfaces potentially lead to reduced electron mobility and distorted morphology. In addition, to achieve the desired growth is difficult since accurate control of the thickness of the layers is required to obtain a required average index of refraction.
Another solution to obtain a large vertical farfield is a spot-size converter as disclosed by M. Wada et al in “Fabrication and coupling-to-fibre characteristics of laser diodes integrated with a spot-size converter having a lateral taper” IEE Proceedings in Optoelectronics, V. 144, No. 2, April 1997, pp 104–108. This solution uses the effect of instabilities associated with the guiding of the vertical mode in asymmetric structures with additional waveguides to suppress the guided mode towards the substrate into a large additional waveguide below the active waveguide. This has to be done adiabatically and requires additional processing or growth steps. In addition, the spot-size converter as part of a laser diode is passive, making such a device less efficient for use as a high power laser diode.
A further solution consists in providing an anti-resonant reflecting optical waveguides (ARROW). Such a solution is described by M. Galarza et al in “Mode-expanded 1.55-/spl mu/m InP-InGaAsP Fabry-Perot lasers using ARROW waveguides for efficient fiber coupling”, IEEE Journal on Selected Topics in Quantum Electronics, V. 8, No. 6, November/December 2002, pp 1389–1398, and by A. M. Kubica in “Design of antiresonant reflecting optical waveguides with an arbitrary refractive index profile core layer” at the Lasers and Electro-Optics Society Annual Meeting 1994, LEOS '94 Conference Proceedings. IEEE, V. 2, 31 October-3 November 1994, pp 63–64. Such vertical ARROW structures have been realized in glass as well as in semiconductor materials. In these structures, the relevant layers need to have specific dimensions so that the mode experiences a resonance. These required dimensions restrict the design possibilities and render ARROW structures unusable for many practical purposes. In addition, ARROW structures show in practice only limited performance, mostly due to the fact that the resonance condition needs to be obtained for the whole operating regime while the enhanced, i.e. the “guided”, mode is basically a lossy mode.
Generally, a “high slope efficiency” of a laser, i.e. a high radiation output vs. current input, obviously requires low internal losses to reach high output powers before the output power starts to get degraded due to thermal effects caused by heating. On the other hand, efficient heat removal requires a large chip size, which unfortunately reduces the slope efficiency since less light can exit the cavity. Thus, the internal losses of a laser and how to minimize them play not only a dominating role in laser design today, but actually limit the achievable power output. Knowing that, the more important it becomes to use the laser's output most efficiently and avoid any unnecessary losses. Here, a laser according to the invention, by providing a reduced vertical farfield, significantly improves the coupling efficiency between the laser and the fiber and therefor represents an important step forward in laser design.
Thus, it is the main and principal object of the invention to provide a solution to the problems addressed further above and overcome the disadvantages and limitations of the described prior art designs. This is achieved by devising a simple and reliable high power laser structure whose manufacturing is easy and offers a high yield. Such a laser, to be useful especially for pump lasers in optical fiber communication systems, e.g. high power EDFAs and Raman pump laser designs, must provide a stable output and, at the same time, must show leading edge performance.